This invention relates to audiovisual signaling devices for use on emergency vehicles, at construction sites, in burglar-alarm and fire-alarm systems, etc., and, more particularly, to a novel and highly effective audiovisual signaling device that performs better but is simpler and less expensive than similar devices known heretofore.
Audiovisual signaling devices are very important in many applications, especially those having to do with public safety. For example, fire-alarm systems ideally include (and in some jurisdictions are required by law to include) both auditory signals and visual signals in order to maximize the probability that they will attract attention in an emergency. Obviously, those who are nearly or totally without sight respond uncertainly or not at all to visual signals, and those who are nearly or totally withohut hearing respond uncertainly or not at all to auditory signals. Thus, a certain small fraction of the population is not adequately protected by an emergency signal that appeals solely to sight or solely to hearing. Moreover, even persons with normal sight and hearing are more likely to notice a signal that is both auditory and visual. For example, when the background "noise" (light or sound) is very intense, the visual or auditory part of the signal, as the case may be, is less readily detected. It is not always possible to know in advance whether the background light or sound will be sufficiently low to permit reliable detection of a visual or auditory signal. By providing two such signals "in parallel" so to speak, the chance that persons to be protected will respond to at least one of the signals under a wide variety of environmental conditions is improved.
Moreover, even under relatively ideal environmental conditions (low ambient light and sound levels), one with normal sight and hearing may nevertheless fail to respond to a signal that is solely auditory or solely visual, simply because the head is turned in the wrong direction, or because one is asleep, or because one's attention is directed elsewhere.
The advantages of audiovisual signals as compared to signals that are merely auditory or merely visual are well recognized, and numerous audiovisual signaling devices are known. However, all audiovisual signaling devices known heretofore have certain drawbacks.
A horn has much to recommend it as a means for producing the auditory portion of the signal, since it produces a strong signal in relation to the power consumed and is inexpensive and rugged. However, horns vibrate in operation to such an extent that they may damage nearby structure, particularly the (necessarily fragile) filament of an incandescent bulb that may be used as a means for producing the visual portion of the signal. The likelihood of filament rupture is even greater when the signaling device is used on mobile equipment such as a police car, fire truck, ambulance, bulldozer, or crane. Moreover, the arcing associated with the repeated opening of the horn contacts must be suppressed by special circuitry or else will cause radio-frequency interference (RFI) and pitting of the contacts.
A xenon flashtube has much to recommend it as a means for producing the visual portion of the signal, since it also produces a strong signal in relation to the power consumed and is inexpensive per se and rugged enough to withstand the shocks typically encountered during service on mobile equipment and the vibrations of even the most powerful commercial horns. However, it is necessary, if a xenon flashtube is used, to provide a very high starting voltage Heretofore, this has required the incorporation of relatively complex and expensive circuitry.